Radiography: Projection Terminology

Projection Terminology

• The human body is a complicated structure, and errors in radiographic positioning or diagnosis can easily occur unless practitioners have a common set of rules that are used to describe the body and its movements.
• Patient aspect terminology:
  • Anterior aspect: the front of the patient
  • Posterior (dorsal) aspect: the back of the patient
  • Lateral aspect: the side of the patient
  • Medial aspect: the side of a body part closest to the midline

Positioning Terminology

• Erect: the patient is sitting or standing, with the posterior aspect against the cassette, or the anterior aspect against the cassette, or the right or left side against the cassette.
• Decubitus: the patient is lying down, with the possibility of lying in supine, prone, or lateral decubitus positions.
• Semi-recumbent: the patient is reclining, part way between supine and sitting erect, with the posterior aspect of the trunk against the cassette.

Projection Types

• Antero-posterior (AP): the central ray is incident on the anterior aspect, passes along or parallel to the median sagittal plane, and emerges from the posterior aspect of the body.
• Postero-anterior (PA): the central ray is incident on the posterior aspect, passes along or parallel to the median sagittal plane, and emerges from the anterior aspect of the body.
• Lateral projection: the central ray passes from one side of the body to the other along a coronal and transverse plane.
• Anterior oblique: the central ray enters the posterior aspect, passes along a transverse plane at some angle to the median sagittal plane, and emerges from the anterior aspect.
• Posterior oblique: the central ray enters the anterior aspect, passes along a transverse plane at some angle to the median sagittal plane, and emerges from the posterior aspect.
• Lateral oblique: the central ray enters one lateral aspect, passes along a transverse plane at an angle to the coronal plane, and emerges from the opposite lateral aspect.

Image Formation

• X-rays are produced from a small area within the X-ray tube and diverge outwards from this area.
• As X-rays pass through the body, some are absorbed by organs and structures, while others pass through to the image-forming equipment.
• Image formation is a projection of the object, differing from the way images are formed on the retina or photographic film.

Density and Contrast

• Density refers to the degree of 'blackening' within the image, with greater radiation incident upon the image detector resulting in greater density.
• Contrast is the difference in density between structures of interest within the image, with low-contrast images showing little difference and high-contrast images showing a larger difference.

Hanging Radiographs

• Radiographs should be correctly hung on a view box prior to evaluation, with different procedures for different body parts and projections.
• Torso, vertebral, cranial, and hip radiographs should be hung as if the patient is standing in an upright position.
• Radiographs of other body parts, such as fingers, wrist, and forearm, should be hung accordingly, with the patient's position during the radiograph taken into account.